The present invention relates to a sleeve for a developing roll member which is utilized in a developing means for electrophotography, electrostatic recording, etc. where a magnetic developer is applied onto the surface of the sleeve and conveyed into a developing region to develop an electrostatic latent image.
A developing roll member conventionally used in electrophotography, electrostatic recording, etc. usually has a construction shown in FIG. 2. A permanent magnet member 1 may be a hollow cylindrical sintered magnet such as hard ferrite which is fixed to a shaft 2 with an adhesive. Alternatively, a permanent magnet member 1 consisting of magnetic powder and a binder resin may be molded integrally with the shaft 2 by an insertion injection molding method where the shaft 2 is placed in a cavity of a mold and then a mixture of magnetic powder and a binder resin is injected into the cavity to form the permanent magnet member 1. Incidentally, the permanent magnet member 1 has a plurality of axially extending magnetic poles (not shown) on the outer surface.
An end portion 2a of the shaft 2 penetrates a hole of a flange 3 and is rotatably supported thereby via a bearing 5a and the other end portion 2b of the shaft 2 is received in a recess of a flange 4 in a rotatable manner with a bearing 5b. The flange 4 has a projection 4a in alignment with the shaft 2. A sleeve 6 is placed around the permanent magnet member 1 and fixed to the flanges 3 and 4. The sleeve 6 is made of a non-magnetic material such as aluminum alloy, stainless steel, etc. 7 denotes a sealing member which is fitted between the shaft 2 and flange 3. The permanent magnet member 1 usually has a diameter of 15-60 mm, and a length of 200-350 mm.
With the above construction, the permanent magnet member 1 and sleeve 6 can be relatively rotated (for instance, the permanent magnet member 1 is fixed stationary and the sleeve 6 is rotated in accordance with the rotation of the flange 4) to conduct a development by forming a magnetic brush of developer containing (i) magnetic toner, or (ii) toner and magnetic carrier.
In the above developing roll member, when the sleeve 6 is made of an aluminum alloy, it can be produced at a low cost and the resulting sleeve 6 has an advantage of being light. However, it is likely to wear away, thereby resulting in poor durability. Further, although the surface of the sleeve 6 is usually subjected to a blast treatment to increase roughness, the desirable surface roughness cannot last long because of the above-mentioned poor durability of aluminum. This leads to a decrease in developer-conveying ability of the sleeve.
To solve the above problems, it has been proposed to subject the sleeve to an anodizing treatment so that the surface of the sleeve is hardened, thereby increasing wear-resistance. However, since the hardened surface layer of alumite is insulated, it turns out to be impossible to apply a bias voltage to the sleeve.
In addition, when the sleeve 6 made of an aluminum alloy is in a rotating state relative to the permanent magnet member 1, an eddy current is generated on the surface of the sleeve 6 and the amount of the eddy current increases as the rotation speed becomes greater.
Especially when the permanent magnet member 1 is rotated at such a high speed as 800-1000 rpm, the amount of eddy current generated is large enough to cause problems such as decrease of developing efficiency and increase of temperatures of developing roll components which is due to Joule heat.
On the other hand, when the sleeve 6 is made of a stainless steel, the resulting sleeve 6 has enough hardness, thereby attaining good durability. The developing roll member utilizing a stainless steel sleeve is free from the problems caused by eddy currents even when the permanent magnet member 1 is rotated at a high speed. However, since a stainless steel is twice as expensive as an aluminum alloy, and since the stainless steel is low in machinability, the total production cost of the developing roll member results in much higher than that in the case where an aluminum sleeve is utilized.
Meanwhile, more and more things such as inexpensiveness, lightness, compactness, high performance, etc. have been required to a developing roll member recently. Therefore, the conventional developing roll members utilizing the above kinds of sleeves are not thought satisfactory.